The subject matter disclosed herein relates generally relates to high strength stainless steels. More particularly, it relates to a precipitation-hardened, martensitic, stainless steel suitable for turbine rotating components.
The metal alloys used for rotating components of a gas turbine, particularly the compressor airfoils, including rotating and stationary blades, must have a combination of high strength, toughness, fatigue resistance and other physical and mechanical properties in order to provide the required operational properties of these machines. In addition, the alloys used must also have sufficient resistance to various corrosion damage due to the extreme environments in which turbines are operated, including exposure to various ionic reactant species, such as various species that include chlorides, sulfates, nitrides and other corrosive species. Corrosion can also diminish the other necessary physical and mechanical properties, such as the high cycle fatigue strength, by initiation of surface cracks that propagate under the cyclic thermal and operational stresses associated with operation of the turbine.
Various high strength stainless steel alloys have been proposed to meet these and other requirements, particularly at a cost that permits their widespread use. For example, U.S. Pat. No. 3,574,601 (the “'601 patent”) discloses the compositional and other characteristics of a precipitation hardenable, essentially martensitic stainless steel alloy, now known commercially as Carpenter Custom 450, and focuses on corrosion resistance and mechanical properties of this alloy. Ultimate tensile strengths (UTS) of 143-152.5 ksi (about 986-1050 MPa) in the annealed (1700-2100° F. (926-1148° C.) for 0.5-1 hour) or non-aged condition are reported for the alloy compositions described in the patent. The literature regarding this alloy reports an aging temperature range for precipitation hardening of about 800 to 1000° F. (about 427 to 538° C.) for 2-8 hours, with aging at about 900° F. (about 480° C.) producing the maximum strength but lowest fracture toughness. The literature also reports a UTS of greater than 175 ksi (1200 MPa) after aging at 900 to 950° F. (about 480 to about 510° C.). The Custom 450 alloy contains chromium, nickel, molybdenum and copper, as well as other potential alloying constituents such as carbon and niobium (columbium), to yield an essentially martensitic microstructure, having small amounts of less than 10% retained austenite and 1-2% or less of delta ferrite. Niobium may be added at a weight ratio of up to 10 times relative to carbon, if carbon is present in an amount above 0.03 weight percent. The alloys were tested for resistance to boiling 65% by weight nitric acid, room temperature sulfuric acid and hydrogen embrittlement and found to have superior resistance to 300 series and other 400 series stainless steel alloys.
In another example, U.S. Pat. No. 6,743,305 (the “'305 patent”) describes an improved stainless steel alloy suitable for use in rotating steam turbine components that exhibits both high strength and toughness as a result of having particular ranges for chemistry, tempering temperatures and grain size. The alloy of this invention is a precipitation-hardened stainless steel, in which the hardening phase includes copper-rich intergranular precipitates in a martensitic microstructure. Required mechanical properties of the alloy include an ultimate tensile strength (UTS) of at least 175 ksi (about 1200 MPa), and a Charpy impact toughness of greater than 40 ft-lb (about 55 J). The '305 patent describes a precipitation-hardened, stainless steel alloy comprising, by weight, 14.0 to 16.0 percent chromium, 6.0 to 7.0 percent nickel, 1.25 to 1.75 percent copper, 0.5 to 1.0 percent molybdenum, 0.03 to 0.5 percent carbon, niobium in an amount by weight of ten to twenty times greater than carbon, the balance iron, minor alloying constituents and impurities. Maximum levels for the minor alloying constituents and impurities are, by weight, 1.0 percent manganese, 1.0 percent silicon, 0.1 percent vanadium, 0.1 percent tin, 0.030 percent nitrogen, 0.020 percent phosphorus, 0.025 percent aluminum, 0.008 percent sulfur, 0.005 percent silver, and 0.005 percent lead.
While the precipitation hardenable, martensitic stainless steels described above have provided the corrosion resistance, mechanical strength and fracture toughness properties described and are suitable for use in rotating steam turbine components, these alloys are still known to be susceptible to both intergranular corrosion attack (IGA) and corrosion pitting phenomena. For example, stainless steel airfoils, such as those used in the compressors of industrial gas turbines, have shown susceptibility to IGA, stress corrosion cracking (SCC) and corrosion pitting on the surfaces, particularly the leading edge surface, of the airfoil. These are believed to be associated with various electrochemical reaction processes enabled by the airborne deposits, especially corrosive species present in the deposits and moisture from intake air on the airfoil surfaces. Electrochemically-induced intergranular corrosion attack (IGA) and corrosion pitting phenomena occurring at the airfoil surfaces can in turn result in cracking of the airfoils due to the cyclic thermal and operating stresses experienced by these components. High level of moisture can result from use of on-line water washing, fogging and evaporative cooling, or various combinations of them, to enhance compressor efficiency. Corrosive contaminants usually result from the environments in which the turbines are operating because they are frequently placed in highly corrosive environments, such as those near chemical or petrochemical plants where various chemical species may be found in the intake air, or those at or near ocean coastlines or other saltwater environments where various sea salts may be present in the intake air, or combinations of the above, or in other applications where the inlet air contains corrosive chemical species. Due to the significant operational costs associated with downtime of an industrial gas turbine, including the cost of purchased power to replace the output of the turbine, as well as the cost of dismantling the turbine to effect repair or replacement of the airfoils and the repair or replacement costs of the airfoils themselves, enhancements of the IGA resistance or pitting corrosion resistance, or both, have a significant commercial value.
In view of the above, stainless steel alloys suitable for use in turbine airfoils, particularly industrial gas turbine airfoils, in the operating environments described and having improved resistance to IGA, or corrosion pitting, or preferably both of them, are desirable and commercially valuable, and provide a competitive advantage.